An Two-Component System Ortholog Regulates Phase Variation, Osmotic Tolerance, Motility, and Virulence in Acinetobacter baumannii Strain AB5075.

Recently, a novel phase-variable colony opacity phenotype was discovered in strain AB5075, where colonies interconvert between opaque and translucent variants. Opaque colonies become mottled or sectored after 24 h of growth due to translucent variants arising within the colony. This easily distinguishable opaque-colony phenotype was used to screen for random transposon insertions that increased the frequency of sectoring at a time point when wild-type colonies were uniformly opaque. A colony was identified that contained multiple papillae of translucent variants, and the insertion in this mutant mapped to an ortholog of the two-component system response regulator Subsequent investigation of in-frame deletions of and the sensor kinase (located adjacent to ) showed that the switching frequency from opaque to translucent was increased 401- and 281-fold, respectively. The mutant also exhibited sensitivity to sodium chloride in growth medium, whereas the mutation did not elicit sensitivity to sodium chloride. Mutation of either gene reduced motility in strain AB5075, but a mutation in both and produced a more profound effect. The and genes were cotranscribed but were not subject to autoregulation by OmpR. Both and mutant opaque variants were attenuated in virulence in the infection model, whereas mutation of had no effect on the virulence of the translucent variant. is a well-known antibiotic-resistant pathogen; many clinical isolates can only be treated by a very small number of antibiotics (including colistin), while some exhibit panresistance. The current antimicrobial arsenal is nearing futility in the treatment of infections, and new avenues of treatment are profoundly needed. Since phase variation controls the transition between opaque (virulent) and translucent (avirulent) states in , this may represent an "Achilles' heel" that can be targeted via the development of small molecules that lock cells in the translucent state and allow the host immune system to clear the infection. A better understanding of how phase variation is regulated may allow for the development of methods to target this process. The two-component system ortholog negatively regulates phase variation in , and perturbation of this system leads to the attenuation of virulence in an invertebrate infection model.